In making lightweight concrete, an important constituent is lightweight aggregate. The most commonly used lightweight aggregate comprises expanded clay, slag or shale for primary use in structural concrete and expanded perlite or vermiculite for primary use in insulating concrete. Mixtures of the two general types of aggregates are sometimes used to obtain a desired strength, bulk density of thermal conductivity. Standards have been established for lightweight aggregate for structural concrete ASTM Specification C 330-64T and for insulating concrete ASTM Specification C 332.
At the present time, the cost of lightweight aggregate is quite high and makes lightweight concrete more costly than conventional concrete.
Accordingly, among the objects of the present invention are to provide a lightweight aggregate utilizing waste materials that can be converted to a form useful for concrete with a minimum expenditure of energy and labor.
Basically, the invention comprises pelletizing cement kiln dust or a mixture of cement kiln dust and fly ash, both of which are waste products, thus with a zero energy cost and subjecting them to a moderate or ambient heat treatment, requiring much less energy than normally required to produce lightweight aggregate, and at high to 100% relative humidity to produce a lightweight aggregate that is useful for making concrete. When used to make lightweight concrete, the resultant concrete product comprises a plurality of pellets interspersed throughout the concrete wherein the pellets are not only physically embedded in the concrete but, in addition, are bonded to the cement through an interaction between the cement and the surface zones of the pellets.
It has heretofore been suggested that a lightweight aggregate might be made by pelletizing fly ash alone and subjecting the fly ash to a sintering. However, such a process now requires substantial additional energy in the sintering. Paper titled "Fly Ash Pelletizing" presented at Annual AIME Meeting, Feb. 27-Mar. 6, 1966, by Donald C. Violetta and Carl J. Nelson.
In England, lightweight aggregate is currently produced from fly ash by pelletizing the dust in a pan pelletizer and then sintering the pellets in a traveling grate furnace. Residual carbon in the ash provides part of the fuel required. The product has a bulk density of 50 to 60 pcf and is used for structural concrete. Reference Rock Products, June, 1975, pp. 69-70, 75.
The levels of carbon found in fly ash nationally in the USA have been dropping significantly in recent years as a direct result of efforts to improve combustion efficiency in thermal power plant systems burning coal, thus saving energy.
It has also heretofore been suggested that cement kiln dust be formed into large briquettes or pellets by compression for recycling in the cement kiln. Article from Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978, pages 468-472. However, I am not aware of any formation of pellets in accordance with the invention or their use in lightweight concrete.
The solid waste generated by cement manufacture is primarily kiln dust. This dust contains a mixture of raw kiln feed, partly calcined material, finely divided cement clinker and alkali sulfates (usually sulfates). There is economic value in returning the dust to the kiln, but when the alkali content of the returned dust is too high for the product clinker to meet specifications, the dust must be discarded. Up to about 15% of the raw materials processed may be collected as dust and of this about half may be low enough to alkalis to be returned to the kiln. The rest is usually stockpiled as a waste material which must be disposed and may be a nuisance and possibly a hazard.
Based on measurements made on a variety of cement kiln dusts that are usually disposed, well over 90% by weight of the dust will pass through a 325 mesh Tyler screen.
Weight average particle diameter is most often of the order of 10 microns.
Although the chemical reactions occurring in the resultant cement kiln dust are not well known, typical cement kiln dust has a chemical analysis as follows:
SiO.sub.2 PA1 Al.sub.2 O.sub.3 PA1 Fe.sub.2 O.sub.3 PA1 CaO PA1 MgO PA1 SO.sub.3 PA1 Na.sub.2 O PA1 K.sub.2 O PA1 Loss Ignition
More specifically, typical cement kiln dust may have the following analyses by weight:
__________________________________________________________________________ Kiln Dust Ign. Sample SiO.sub.2 Al.sub.2 O.sub.3 Fe.sub.2 O.sub.3 CaO MgO SO.sub.3 Na.sub.2 O K.sub.2 O Loss __________________________________________________________________________ 1 13.30 3.75 1.57 36.85 1.26 7.97 0.44 9.08 26.38 2 14.00 4.45 1.61 41.56 2.74 5.16 2.06 4.22 25.48 3 15.32 2.67 3.39 42.07 1.01 2.76 1.01 6.03 27.08 4 16.08 5.36 2.22 39.66 2.29 0.27 0.17 1.51 7.14 5 14.66 3.88 1.37 47.98 0.54 4.42 3.61 3.62 21.06 6 10.92 3.87 1.49 36.95 1.46 8.74 0.36 14.34 21.42 7 11.20 3.20 1.40 48.80 2.10 2.40 0.20 4.20 26.60 8 13.86 3.91 1.69 42.87 1.44 8.53 1.82 4.58 20.08 9 13.00 4.00 5.00 47.20 1.20 13.60 0.45 2.90 12.90 10 17.12 6.26 2.18 49.09 1.12 3.26 2.43 3.92 17.10 11 15.66 5.18 2.34 40.30 2.11 6.30 1.54 8.44 19.40 12 16.95 5.14 2.18 40.30 1.16 7.37 3.64 18.08 21.98 13 11.00 4.08 2.02 40.66 0.54 9.33 1.48 9.08 20.22 14 16.34 4.36 2.18 46.95 0.58 3.74 3.92 3.77 21.12 15 14.20 4.88 2.58 42.19 0.85 7.23 1.54 7.53 20.04 16 14.48 5.00 2.02 52.61 0.90 2.74 1.21 1.72 21.05 17 16.70 4.44 2.02 47.10 1.82 1.82 0.10 2.11 24.67 __________________________________________________________________________
______________________________________ Range of Specific Elements (as Oxides) Low % High % ______________________________________ SiO.sub.2 10.92 17.12 Al.sub.2 O.sub.3 2.67 6.26 Fe.sub.2 O.sub.3 1.37 3.39 CaO 36.85 52.61 MgO 0.54 2.74 SO.sub.3 0.27 13.60 Na.sub.2 O 0.10 3.92 K.sub.2 O 1.51 18.08 Ignition Loss 7.14 27.08 ______________________________________
When mixtures made in accordance with the invention and mixed with water to produce a pozzolanic reaction have been tested in accordance with the specifications given in ASTM C-593 for fly ash and other pozzolans for use with lime, it has been found that the compositions meet or exceed the specifications.
The term "fly ash" as used in connection with stabilized bases is well known and as used herein is intended to indicate the finely divided ash residue produced by the combustion of pulverized coal or lignite, which ash is carried off with the gases exhausted from the furnace in which the coal is burned and which is collected from these gases usually by means of suitable precipitation apparatus such as electrical precipitators. Those finely pulverized ashes resulting from combustion of oil and from combustion of waste materials in a large incinerator or natural pozzolans can also be utilized in the methods described herein providing their chemical compositions are reasonably similar to pulverized coal fly ashes. The fly ash so obtained is in a finely divided state such that usually at least 70% by weight passes through a 200-mesh sieve, although incinerator ashes may be considerably coarser. Such fly ash may be considered an "artificial pozzolan", as distinguished from a "natural pozzolan". Typical pozzolanic fly ashes comprise those known as western fly ash, sub-bituminous fly ash and lignitic fly ash.
In accordance with the invention, cement kiln dust from the cement kiln, alone or in a mixture with fly ash is pelletized preferably in a shallow pan pelletizer of well-known construction to produce pellets ranging in size from about 60 mesh Tyler sieve series to over 3/4-inch diameter. Shallow pan (disc) pelletizers are normally preferred for producing fine pellets, while deep drum pelletizers are preferred for making large pellets. For concrete applications, a wide range of particle sizes proportioned in a smooth grading is desirable for optimum workability and strength. Although pelletizers typically produce pellets of a relatively narrow size range at any set of process parameters, a range of pellet sizes can be made by varying the process parameters (particularly water content and feed location) to produce a desired size range. Pellets with different size ranges can then be blended together to produce a wide size range if desired.
Further, in accordance with the invention, small amounts of additives may be added to enhance the physical properties or the handling properties. For example, small amounts up to 10% of additives to increase the strength or rate of increase could be used such as portland cement, quick lime, free lime from any source, or water soluble salts such as sulfates, carbonates and hydroxides of sodium and potassium and mixtures thereof such as alkali metal, alkaline earth metal sulfate and acid sulfate such as sodium, potassium and magnesium sulfate. In addition, small amounts of additives such as water reducers, other wetting agents commonly utilized in concrete-making may be used which reduces the amount of water necessary to achieve strength development.
The resultant pellets are then subjected to a low temperature hydrothermal treatment resulting in a lightweight aggregate that has sufficient strength to serve for use in making concrete.
More specifically, the pellets are subjected to heat and high relative humidity which includes 100%. Those skilled in the arts can device various low cost methods for controlling both temperature and humidity of large volume of pellets.
It has been found that water utilized during the pelletizing should be approximately 20 to 35 percent (based on dry material weight) for kiln dust alone or for mixtures of cement kiln dust and fly ash. However, the water content will vary with the characteristics of the particular cement kiln or fly ash dusts being used, and may be higher or lower than the desirable range identified above for cement kiln dust alone or for mixtures of cement kiln dust and fly ash.